energetics

Question 1

what are the standard conditions defined for thermochemistry?

Answer 1

298K (temperature)
1 bar/100000 Pa (pressure)
1 mol/dm3 (concentration of any solution)

Question 2

what is standard enthalpy change of formation (ΔHf)?

Answer 2

the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 298K and 1 bar

Question 2

what is the standard enthalpy change of reaction?

Answer 2

the enthalpy change when molar quantities of reactants as specified by the chemical equation react to form products at 298K and 1 bar

Question 3

what is standard enthalpy change of combustion (ΔHc)?

Answer 3

the heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298K and 1 bar

Question 4

what is standard enthalpy change of neutralisation (ΔHneut)?

Answer 4

the heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base at 298K and 1 bar.

Question 5

what is the formula for heat change of solution?

Answer 5

q = mcΔT
where q = heat change of solution (J)
m = mass of solution (g) = vol x density
c = specific heat sapacity of solution (J g⁻¹ K⁻¹)
T = change in temperature of the solution (K or ⁰C)
OR
q = CΔT
where q = heat change of solution (J)
C = heat capacitiy of solution (J K⁻¹)
T = change in temperature of the solution (K or ⁰C)

Question 6

how to calculate enthalpy change?

Answer 6

endothermic: +q/n
exothermic: -q/n

where n = no. of moles of substance according to the definition of the enthalpy change

Question 7

what is hess’ law?

Answer 7

the enthalpy change of a reaction is determined only by the initial and final states and is independent of the reaction pathway taken

Question 8

what is bond energy?

Answer 8

bond energy is the energy required to break 1 mole of a covalent bond in the gaseous state

Question 9

how to calculate ΔHr using bond energy?

Answer 9

ΔHr = bond energy of bonds broken - bond energy of bonds formed

ONLY APPLICABLE FOR GASEOUS MOELCULES

Question 10

what is standard enthalpy change of atomisation (ΔHatom)?

Answer 10

the energy required when 1 mole of gaseous atoms is formed from the element at 298K and 1 bar
OR
the energy required to convert 1 mole of the compound into gaseous atoms at 298K and 1 bar

ΔHatom values are ALWAYS POSITIVE (ENDOTHERMIC!) as atomisation involves breaking bonds

Question 11

what is lattice energy?

Answer 11

lattice energy is the heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions.

always exothermic!

Question 12

what is ionisation energy?

Answer 12

the first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cations

I.E. is ALWAYS POSITIVE (ENDOTHERMIC) as energy is required to remove an electron

Question 13

what is electron affinity?

Answer 13

first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous anions

• 1st EA is typically negative
• 2nd EA onwards always positive as energy is required to overcome the electrostatic repulsion between the incoming electron and the anion

Question 14

what is standard enthalpy change of hydration (ΔHhyd)?

Answer 14

the heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar

always negative as heat is evolved in forming ion-dipole interactions

Question 15

what is standard enthalpy change of solution (ΔHsol)?

Answer 15

the enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at 298K and 1 bar

ΔHsol = -L.E. + ΔHhyd
ΔHsol highly positive - salt is likely insoluble
ΔHsol negative - salt is likely soluble

Question 16

what is entropy (S)?

Answer 16

entropy is a measure of the randomness or disorder in a system, reflected in the number of ways that the energy of a system can be distributed through the motion of its particles

ΔS > 0 - more ways to distribute energy
ΔS < 0 - less ways to distribute energy

Question 17

how does ΔS change with temperature?

Answer 17

• as temperature increases, average kinetic energy of the particles and the range of energies increase
• there are more ways to disperse the energy among the particles, so entropy increases.

Question 18

how does ΔS change with state?

Answer 18

• in solid state, particles vibrate about their fixed positions. the energy is least dispersed and the solid has lowest entropy
• when temperature increases, entropy increases gradually as the kinetic energy of the particles increases
• when the solid melts, particles move more freely in the liquid state and become more disordered, so there is an abrupt increase in entropy as there are more ways to distribute the particles and their energy in the liquid state
• when a liquid is heated to a gas, the particles are able to move even more freely, so there are more ways to distribute the particles and their energy in the gaseous state

Question 19

how does ΔS change with number of particles (especially gas)?

Answer 19

increase in no. of gas particles = particles are more disordered = more ways the particles and the energy can be distributed = increase in entropy

Question 20

how does ΔS change when gaseous particles mix?

Answer 20

volume available for each gas increases = more ways to distribute particles and their energy = entropy increases

BUT if gases are mixed at constant volume, volume available to distribute each gas particle is the same and entropy does not change

Question 21

how does ΔS change when an ionic solid dissolves in water?

Answer 21

• entropy increases because the ions in the solid are free to move in solution
• entropy decreases as water molecules that were originally free to move become restricted in motion as they arrange themselves around the ions
• if the first factor is more significant, overall ΔS is positive

Question 22

what is gibbs free energy change?

Answer 22

ΔG = ΔH - TΔS

Question 23

how to use the sign of ΔG to predict if a reaction is spontaneous?

Answer 23

ΔG < 0: reaction takes place spontaneously (exergonic)
ΔG = 0: system is at equilibrium, no net reaction in the forward or backward reaction
ΔG > 0: reaction cannot take place spontaneously (endergonic). the reaction is spontaneous in the reverse direction